Creating a window of opportunity for establishing ecosystem engineers by adding substratum: a case study on mussels

Abstract

Ecosystem engineers typically exert positive feedback on their environment, which enhances their performance. Such positive feedback is lacking in the establishment phase, when densities are too low and/or patches are too small. There is a strong need to unravel the mechanisms for overcoming the resulting establishment thresholds, both for ecological restoration purposes and to be able to use their services. In the present study, we question whether providing a transient substratum can be used as tool to overcome establishment thresholds, by creating a window of opportunity for initial settlement, using mussels (Mytilus edulis) as a model system. Combining field and flume experiments, we study how biogenic substratum enrichment in the form of a shell layer on a soft mudflat affects the critical dislodgement thresholds and, thus, the chances of mussel establishment at different mussel densities and aggregation states. Flume results showed that the presence of a shell layer reduced dislodgement of mussel patches in low‐energy environments but was conditional for establishment in high‐energy environments. That is, in high‐energy environments with shells, aggregation into clumps enhanced dislodgement, while dislodgement was reduced with increasing overall mussel biomass and overall mussel patch weight. Without shells, dislodgement was always 100%. These findings agreed with our field studies, which showed that coarse shell material reduced mussel losses (by a factor of 3), reduced aggregation (by a factor of 2.4), and increased attachment strength (by a factor of 2.4). Overall, our results show that the local presence of biogenic substratum increases the chance of mussel establishment by enhancing the critical hydrodynamic dislodgement threshold. Thus, the local addition of a biogenic substratum may create a window of opportunity to initiate settlement in more dynamic environments, to shift at a local scale from a bare mudflat state into an established biogenic reef state. Our findings have clear implications for how to approach restoration and management of ecosystem engineers dominated systems. For instance, when positive feedback of ecosystem engineers is lacking, (1) the transient offering of suitable settling substratum may be a necessary step to overcome establishment thresholds, and (2) this becomes increasingly important with increasing abiotic stress.

Putting self‐organization to the test: labyrinthine patterns as optimal solution for persistence

“…With laboratory experiments simulating realistic flow and wave conditions, we confirmed that labyrinthine spatial patterns can maximize resistance to dislodgement , Liu et al 2013. Previous studies in a similar flume set-up found that clump dislodgement thresholds were directly related to clump mass (Capelle et al 2019). For our experimental design, we did not weigh clumps.…”

“…The formation of self-organized spatial patterns in mussel beds provides a unique, easily accessible model system to study the relation between density-dependent pattern formation and environmental drivers, such as the minimization of individual mortality by decreasing wave-driven dislodgement via the formation of heavy clumps (Capelle et al 2019) and other forms of population loss (e.g. predation).…”

Restoring mussel beds in highly dynamic environments by lowering environmental stressors

“…The complex structure of the oyster bed may have provided refuges, making it more difficult for predators such as crabs or birds to reach them. Thus, the addition of a complex substrate may lower dislodgement caused by both hydrodynamic stress and predators (Bartol & Mann 1997; Schulte et al 2009; Capelle et al 2019). Rough substratum, such as coir‐nets or oyster shells, may also have affected the mussels by reducing local bedload transport of sediment (Commito et al 2018; Commito et al 2019), or by increasing the retention of water and thereby reducing the thermal stress experienced by the mussels during daytime low tides (Helmuth & Hofmann 2001).…”

Are all patterns created equal? Cooperation is more likely in spatially simple habitats

“…In each tank, two out of the four arenas were layered with empty shell material, while the other two offered only the bare substratum of the tank (plastic). These substrata were chosen to represent a simple hard substratum where movement is maximised but attachment is limited (bare plastic tank) and a complex hard substratum where attachment is possible and often preferred (mixed cockles and mussels shell material, in pieces > 3cm, Capelle, Leuchter, Wit, Hartog, & Bouma, 2019, Bertolini, Geraldi, Montgomery, & Connor, 2017). Fifty mussels of either small (20.52 ± 0.61mm, mean ± SE ) or large (38.75 ± 0.54mm, mean ± SE) size classes were then added and spread evenly in one of the arenas (chosen at random) to obtain a density of 200 mussels/m 2 , representative of a natural lowmedium density (Bertolini et al, 2019), and were left for two hours to form byssal attachment.…”